Introduction

By redistributing heat over the globe, ocean currents have a major impact on the global climate. They cause the relative mildness of the Western European climate, for example. Ocean and atmospheric currents form a coupled dynamic system. Instabilities of this system, the El Nino Southern Oscillation (ENSO) in particular, produce important climate fluctuations. Ocean currents not only distribute heat, but they also play a crucial role in the global ecosystem by storing CO2 and recycling nutrients.

Currents

There are two main types of ocean currents: currents driven mainly by wind and currents mainly driven by density differences. Density depends on temperature and salinity of the water. Cold and salty water is dense and will sink. Warm and less salty water will float. Although tidal currents are most prominent in shallow coastal waters, they are of minor importance in the oceans. It should be noted, however, that tides are mainly generated in the oceans (by the gravitational forces of moon and sun) and are amplified when propagating onto the continental shelf.

Wind-driven ocean currents

Global wind field

The large-scale global wind field consists of dominating westerly winds at latitudes between 30 and 60 degrees in the northern and southern hemispheres (the Westerlies) and dominating easterly winds in the tropical/subtropical zone (the Trade winds). This wind field pattern results from the low atmospheric pressure in the tropics (warm ascending air) and high atmospheric pressure in the subtropics (cooled descending air). The near-surface air flow to the equator at low latitudes and to the poles at high latitudes, resulting from these so-called Hadley cells, is deflected by earth rotation, hence giving rise to the Westerlies and the Trade winds.

Surface currents

Wind stress generates strong currents (up to several m/s) in the ocean surface layer. The thickness of the surface layer entrained by wind is of the order of 500 meters (about the thickness of the thermocline at low- and mid-latitudes), up to a maximum of 2000 m. Due to earth rotation the main ocean current system consists of large anticyclonic gyres (clockwise rotating in the northern hemisphere and anticlockwise in the southern hemisphere) [1]. There are five major gyres: the North Atlantic, the South Atlantic, the North Pacific, the South Pacific and the Indian Ocean Gyre, see figure 1.
The Antarctic Circumpolar Current is situated in the Southern Ocean and constantly circles around Antarctica because there are no land masses to interrupt the currents. It is an eastward-flowing current driven by the dominant western winds at this latitude.

The most famous ocean current, the Gulf Stream, is a vast moving mass of water, transporting an enormous amount of heat from the Caribbean across the ocean to Europe. It passes by the US east coast as a narrow jet, due to the northward increase of the Coriolis effect[3] and then spreads out as a meandering current over the ocean while generating a series of meso-scale eddies and whirls. The North Atlantic Gyre is completed by the Canary Current in the Eastern Atlantic that transports relatively cold water south and west. The Kuroshio is a warm boundary current in the north-western Pacific, similar to the Gulf Stream. It is part of the large gyre formed by the California Current and the North Equatorial Current. The North Equatorial Current and South Equatorial Current are driven by the easterly trade winds over the Pacific. The Southern Pacific Gyre is completed by the warm West Australian Current and the cold Peru Current.

Upwelling

In regions where Ekman transport deflects the boundary current from the coast, water from the deep ocean rises to the ocean surface, see figure 2. This phenomenon is called 'upwelling' and is very important for enrichment of surface waters with organic matter and nutrients. Upwelling zones are characterized by a very rich marine life with abundant resources for fishery. Upwelling zones exist at the southward flowing boundary currents in the Northern Hemisphere (California Current along the US West Coast, Canary Current along the West African coast) and at the northward flowing boundary currents in the Southern Hemisphere (Peru Current along the South American West Coast and Benguela Current along the South African West Coast).

Upwelling also occurs at the equator at the Pacific Ocean (Equatorial upwelling). The North Equatorial Current is deflected to the north and the South Equatorial current to the south as a consequence of the Coriolis effect. This produces upwelling of nutrient rich water and cooling of the surface water near the equator of the Pacific, see figure 3. Downwelling zones exist north and south of the equator.

El Nino Southern Oscillation

Instability of the coupled ocean-atmosphere dynamics produces large fluctuations in the climate of the Pacific region, which are felt at the global scale. Weakening of the easterly trade winds allows warm water from the Western Pacific to flow back with the Equatorial Counter Current to the eastern South American boundary, where upwelling currents of cold deep ocean water are shut off. This results in relative warming of the Eastern Pacific (lowering the sea surface atmospheric pressure) and relative cooling of the Western Pacific (increasing the sea surface atmospheric pressure) and hence induces a further weakening of the easterly trade winds. This feedback strengthens the so-called El Nino phase of the oscillation [5][6]. The shut-off of the food-rich upwelling currents has major consequences for marine life and fisheries. [7]. After a number of years (three on average, but variable) the system sweeps back to the opposite phase, called El Nina. The onset and offset of the oscillation are still not fully understood.

Deep ocean circulation

Deep ocean circulation is primarily driven by density differences. It is called thermohaline circulation, because density differences are due to temperature and salinity. Density differences are small and the flow velocity is low, of the order of a few cm/s. However, the water masses moving around by thermohaline circulation are huge. Water fluxes are of the order of 20 million m3/s. Density gradients alone are not sufficient for sustaining the deep ocean circulation. Upwelling and mixing processes, to bring deep ocean water back to the surface, are required too [8].

Deep water formation

The density of surface water increases when frigid air blows during winter across the ocean at high latitudes. The water density increases further by evaporation and by salt expulsion when sea ice is formed. Deep ocean water masses are formed in the Arctic and Antarctic regions by sinking of dense water with a temperature less than 4°C from the surface to great depth. From these regions, a cold deep water layer spreads over the entire ocean basins.

Conveyor belt

The thermohaline circulation moves water masses around between the different ocean basins [9][10]. 'Ocean conveyor belt' is the popular name of this inter-basin circulation. The conveyor belt is fed in the northern North Atlantic with high-salinity water (due to evaporation) supplied by the Gulf Stream, which sinks to great depth after cooling down in the Arctic region, forming the North Atlantic Deep Water (NADW). The replacement of this dense sinking water generates a continuous surface flow feeding the conveyor belt. The NADW flows from the Arctic region southward, as a deep boundary current along the American shelf [11]. This current compensates for the net northward surface flow in the Atlantic Ocean. This circulation along the north-south axis is called Atlantic Meridional Overturning Circulation (AMOC) , see Fig. 4.

The NADW finally joins the Antarctic Circumpolar Current and enters the Indian and Pacific oceans. The cold dense water from the Antarctic zone fills the deep water layer in these oceans and then gradually rises and mixes with the surface waters of the Indian and Pacific oceans. The mixing of deep ocean water is promoted by strong surface winds, by tides, by upwelling and by abyssal circulation[12][8]. The circulation is finally completed by a warm surface return current to the Atlantic Ocean that passes south of Africa and America, see figure 5. The whole trip takes more than 1,000 years to complete.

Importance of deep ocean circulation

The deep ocean is a huge storehouse of heat, carbon, oxygen and nutrients. Deep ocean circulation regulates uptake, distribution and release of these elements. The low overturning rate stabilizes our global climate. By carrying oxygen into the deeper layers it supports the largest habitat on earth.

Deep ocean circulation and climate change

Present theories for explaining the deep ocean circulation predict that global warming will have a negative impact on the deep ocean circulation. Most studies have focused on the northern Atlantic [13]. The formation of dense sinking surface water in the Arctic region will be counteracted by a higher atmospheric temperature and by release of fresh water by ice melting. The feeding of the Atlantic Meridional Overturning Circulation, which drives warm Gulf Stream waters to the north, will thus be reduced. Besides, the density of the North Atlantic Deep Water will be lower; therefore the cold return current will flow closer to the ocean surface. It is expected that these factors will cause significant cooling of the West European climate.

Ice melting and resulting fresh water releases in the Antarctic region will hamper the formation of Antarctic Bottom Water (AABW). Model simulations indicate that this may entail considerable warming of deep waters in the entire Pacific; it may also affect the Atlantic by strengthening the Atlantic Meridional Overturning Circulation. The impact of fresh water releases in the Antarctic region on the global climate and sea-level rise may even be greater than the impact of freshening of the Arctic waters, as discussed in the article Thermohaline circulation of the oceans.